A Recombination Model for Ga As Solar Cells
A Recombination Model for Ga. As Solar Cells Keyuan Zhou and Tim Gfroerer, Davidson College Yong Zhang, UNC Charlotte Abstract Solar cells convert sunlight into electricity. But defects in solar cells are one of the major factors inhibiting conversion efficiency. Defects allow for the recombination of charge carriers, so they fail to contribute to the electrical output. Measurements and preliminary analysis by Ashley Finger (’ 14) show illumination and temperature-dependent trends in Ga. As that help clarify the role of defects. My research aims to develop a new way to analyze the data. In particular, I seek to improve the model describing the recombination process under the influence of defects in the semiconductor material. Previous Experiments What is a Ga. As Solar Cell? Interface (macroscopic defect) region Ga. In. P Upper Confinement Layer Bulk defect region Ga. As Active Layer Ga. In. P Fig 5 a: Ga. As sample diagram Lower Confinement Layer • Ashley Finger (’ 14) and Dr. Gfroerer made Fig 5 b: Experimental setup diagram from thesis file of Ashley Finger (’ 14) prior measurements on a Ga. As sample Fig 1: Picture of a solar cell and diagram of its working mechanism • A device that converts light energy directly into electricity • By shining a laser on the sample, electron-hole pairs are generated, and a camera and oscilloscope are used to study the recombination process • Ga. As is a crystalline compound of elements gallium and arsenic Our Study and Preliminary Model • High efficiency but high cost! What are Defects? • Our focus is the recombination process of the electrons and holes around a macroscopic defect. MODEL • Previous work included measurements of response under different illumination at different temperatures ranging from 77 K-295 K • We have constructed a physical model and we compare theoretical curves with the experimental results Fig 2 a: Illustration of macroscopic defects • Occur during the manufacturing process • Statistically, defects are unavoidable • Microscopic or bulk defects are misplaced or alternative atoms in the crystal • Macroscopic defects are extended mismatch features in the crystalline structure Fig 2 a: Illustration of microscopic defects Carrier Generation and Recombination • Electrons absorb the energy from a photon and jump to a higher energy level Energy Conduction Band Photon in Photon out Valence Band Electrons • The vacancies left behind will have an effective positive charge – these vacancies are called holes Holes Fig 6 a: Radiative efficiency analysis Fig 3 a: Illustration of Carrier generation and recombination • This absorption process is called carrier generation Fig 6 b: Transient analysis of carrier lifetime • When the electrons fall down to the lower energy level, a photon may be re-emitted • Diffusion lengths are calculated by the equation where • The electron fills the hole (a process called recombination) and the time between generation and recombination is called the lifetime • μ is the mobility of electron with literature value of 8500 cm 2 V-1 s-1 • Carriers can diffuse before recombining, and the distance traveled is called the diffusion length • k is the Boltzmann constant and q is the electron charge Note: Initial densities for theoretical calculations over-estimate experimental steady-state conditions Fig 3 b: Illustration of diffusion and laser excitation Why do defects matter? • Energy from the incident light is dissipated as heat • Reduces electrical output • Heat generation can also damage the cell Fig 6 c: Diffusion length analysis, open symbols are theoretical results Working Model and Future Work Late this summer, we discovered a new method that uses the differential equation model to fit the data directly without any assumptions/simplifications Fig 7: Comparison of the transient data fits with the preliminary model (red and green) and working model (dark green) at 239 K Fig 4 a: Thermal picture of a solar panel by Dr. Gfroerer Fig 4 b: Diagram of defect-related loss
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